Combustion engines have a cylinder block and at least one cylinder head, which are connected to one another in order to form the cylinders. The cylinder head is usually used to accommodate the valve gear. In order to control gas exchange, a combustion engine may require control elements, generally in the form of valves, and actuating devices for actuating these control elements. The valve actuating mechanism, which may be inclusive of the valves themselves, required to move the valves is referred to herein as the valve gear. In the course of gas exchange, the combustion chambers are filled with charge-air via the inlet openings.
According to the prior art, the exhaust lines adjoining the outlet ports are at least partially integrated into the cylinder head and may be combined to form a common overall exhaust line or may be combined in groups to form two or more overall exhaust lines. The combination of exhaust lines to form an overall exhaust line may be referred to generally and within the context of the present disclosure as an exhaust manifold.
The way in which the exhaust lines of the cylinders are combined in individual cases, i.e. the specific configuration of the exhaust gas discharge system, may depend primarily on which operating range of the combustion engine has priority, i.e. in respect of which operating ranges the operating behavior of the combustion engine is to be optimized.
In the case of pressure-charged combustion engines in which at least one turbine of an exhaust turbocharger is provided in the exhaust gas discharge system and which may be configured to have a satisfactory operating behavior in the lower engine speed or load range, i.e. in the case of relatively small exhaust gas volumes, “pulse turbocharging” may be the desired operational state.
In this case, the dynamic wave processes taking place in the exhaust gas discharge system, especially during gas exchange, may be used for the purpose of pressure charging as well as in the process of improving the operating behavior of the combustion engine.
The evacuation of the combustion gases from one cylinder of the combustion engine as part of the gas exchange process may be based primarily on two different mechanisms. If the outlet valve opens in a position relatively close to bottom dead center at the beginning of gas exchange, the combustion gases may flow at high speed through the outlet port into the exhaust gas discharge system, owing to the high pressure level prevailing in the cylinder toward the end of combustion and the associated high pressure difference between the combustion chamber and the exhaust line. This pressure-driven flow process may be accompanied by a high pressure peak, herein also referred to as a pre-exhaust surge, which may propagate along the exhaust line at the speed of sound, wherein the pressure may be decreased or reduced to a greater or lesser extent with increasing distance traveled, owing the reduction of pressure mainly to friction.
As gas exchange progresses, the pressures in the cylinder and in the exhaust line may equalize, and therefore the combustion gases may then be expelled primarily due to the stroke motion of the piston, rather than by the effect of pressure.
At low engine speeds, the pre-exhaust surge may be used in a certain selected manner for pulse charging, wherein high pressure pulses of short duration may be used in an effective manner for energy recovery in the turbine. In this way, high boost pressure ratios, such as high boost pressures on the inlet side, may be generated by means of exhaust turbocharging, even with only relatively small exhaust gas volumes, in particular at low engine speeds.
Pulse charging may provide a method in which the acceleration of the turbine wheel responds in a predetermined manner. For example, pulse charging may be used in increasing the turbine speed, which may fall considerably and noticeably to a user during idle operation of the combustion engine or at low load and which may often be raised again as far as possible without delay by means of the exhaust gas flow in the event of an increased load demand. The inertia of the turbine wheel and the friction in the shaft bearing assembly may generally delay acceleration of the turbine wheel to higher speeds of rotation and therefore, may provide an immediate increase in the boost pressure.
Other attempts to address pulse charging include providing a mixed flow turbocharger. One example approach is shown by Uhlenhake et al. in WO 2014099330A1. Therein, a turbocharger having an asymmetric twin scroll volute design is provided. The larger of the two volutes may eliminate the need for a waste gate and the associated actuator.
However, the inventors herein have recognized potential issues with such systems. As one example, in order to be able to use the dynamic wave processes that may take place in the exhaust gas discharge system, in particular the pre-exhaust surges, for pulse charging to improve the operating behavior of the combustion engine, the pressure peaks or pre-exhaust surges must be maintained in the exhaust gas discharge system. It may be particularly useful if the pressure pulses may be intensified in the exhaust lines or at least do not weaken each other or cancel each other out such as in destructive interference. It may be expedient therefore to group the cylinders in such a way or to combine the exhaust lines in such a way that the high pressures, in particular the pre-exhaust surges of the individual cylinders, may be maintained in the exhaust gas discharge system and that the mutual interference may be avoided as effectively as possible.
Pressure-charged combustion engines in which the cylinders are grouped are also within the scope of the present disclosure. According to the present subject matter, at least two cylinders may be configured in such a way that they form two groups, each group comprising at least one cylinder. The exhaust lines of the cylinders of each cylinder group in each case may come together to form an overall exhaust line, thereby forming an exhaust manifold. In this case, the cylinders may be grouped in such a way that the dynamic wave processes in the exhaust lines of the cylinders of a group have as small a negative effect on the other as possible.
In the case of a cylinder head having four cylinders arranged in series, one embodiment may combine two cylinders with an ignition spacing of 360° crank angle into one group. If, for example, ignition in the cylinders is initiated according to the ignition sequence 1-2-4-3 or according to the ignition sequence 1-3-4-2, one example may combine the outer cylinders into a first group and the inner cylinders into a second group.
In the context of cylinder grouping in order to obtain pulse charging, two further aspects must be taken into consideration, these being highly relevant with respect to the separation of the exhaust gas discharge systems of the cylinder groups. On the one hand, it is increasingly often the case that exhaust manifolds are being integrated into the cylinder head in order to participate in a liquid cooling system provided in the cylinder head an in order to avoid having to manufacture the manifolds from materials resistant to high thermal stress, which may be expensive. On the other hand, there may be a fundamental aim of arranging the turbine provided in the exhaust gas discharge system as close as possible to the outlet of the combustion engine such as close to the outlet ports of the cylinders for example. There are several reasons for this placement and each may include its own advantages, in particular because the exhaust gas paths between the cylinders and the turbine may be shortened. Not only may the path of the hot exhaust gases to the turbine be shortened, but the volume both of the individual exhaust manifolds and of the overall exhaust gas discharge system may likewise be reduced. In this way, improved use may be made of the exhaust gas enthalpy which may decisively be determined by the exhaust gas pressure and the exhaust gas temperature. The shortening of the lengths of the lines and the associated reduction in the exhaust gas volume upstream of the turbine wheel may improve the response of the turbine in at least one example embodiment.
However, the fact that the paths from the outlet ports of the cylinders to the turbine wheel of the provided turbine may be significantly shortened according to the concept described above and may also have potential issues. Owing to the arrangement of the turbine relatively close to the engine, the exhaust gas discharge systems of the cylinder groups may not be separated for long enough. Therefore, according to the present disclosure, the two overall exhaust lines of the cylinder groups may be connected to a double-flow turbine, wherein the two flows may be separated from one another, at least in some section or sections, in the direction of the at least one turbine wheel by a housing wall in continuation of the overall exhaust lines, thereby ensuring that the exhaust gas discharge system of the cylinder groups are also separated from one another over a longer distance.
Despite the optimization measures and features described above, pulse charging may require further improvements.
In one example, the issues described above may be addressed by a pressure-charged combustion engine having at least one cylinder head comprising at least two cylinders, in which each cylinder has at least one outlet port for discharging the exhaust gases from the cylinder via an exhaust gas discharge system, and an exhaust line adjoins each outlet port, at least two cylinders are configured in such a way that they form two groups, each comprising at least one cylinder, the exhaust lines of the cylinders of each cylinder group in each case come together to form an overall exhaust line, thereby forming an exhaust manifold, and the two overall exhaust lines are connected in such a way to a double-flow turbine of an exhaust turbocharger, said turbine comprising a turbine wheel mounted on a rotatable shaft in a turbine housing and equipped with rotor blades, that in each case one overall exhaust line is connected to one of the two flows of the turbine, wherein the two flows are separated from one another, at least in some section or sections, as are therefore also the exhaust gas discharge systems of the cylinder groups, in the direction of the turbine wheel by means of a housing wall in continuation of the overall exhaust lines, wherein each rotor blade of the turbine wheel has an inlet edge facing the flows, wherein a first inlet edge associated with a first flow and a second inlet edge associated with a second flow are connected to one another at a connection point and in this way form the inlet edge, which has an inflection at the connection point. In this way, the design and configuration of the rotor blade edge may make it possible to optimize the inlet edges facing the two flows such as the two subsections of the edge which may be associated with the two flows, independently of one another in respect of the associated flow, more specifically in respect of pulse charging.
As one example, a rotor blade edge of this kind may take into account the fact that the two flows generally comprise different geometrical configurations and therefore may also have different gas dynamics. Thus, the two flows may require differently shaped inlet edges in one embodiment.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.